The present invention relates to a wellhead assembly, for example a wellhead completion assembly, and to a horizontal tree (also referred to as a Spool Tree™) for use in such an assembly. The invention is particularly concerned with assemblies for use in subsea wellhead installations.
It is well known in the art of oil and gas exploration and production for wells to be cased and suspended from a wellhead housing. Once the well has been completed and production is to begin, it is conventional practice to install a so-called Christmas tree on the wellhead housing, the Christmas tree having a bore therethrough in communication with the well and through which fluids from the well are produced. Conventionally, the Christmas tree is provided with one or more vertical bores. Access to the well while the Christmas tree is in place is provided by the one or more vertical bores. However, the access for downhole tools to enter the well is limited by the diameter of the vertical bore or bores in the Christmas tree. In the past, this has served as a major constraint to the downhole operations that may be performed with the Christmas tree in place on the wellhead housing.
These and other problems of conventionally designed Christmas trees and wellhead assemblies were overcome by the introduction of a horizontal tree or so-called Spool Tree. In the horizontal tree, production fluids leave the central bore of the tree through a port extending laterally through the tree wall. Should access be required to the well during production, tools may be passed through the central bore, for example through a conventional blowout preventer (BOP) located on top of the horizontal tree. Due to the relatively larger diameter of the bore of the horizontal tree, access to the well through the tree is significantly improved, compared with the conventional vertical tree design. This obviates the need to remove the tree when downhole operations are required, a dangerous task, in particular when dealing with subsea wellhead installations which may be a considerable distance below the surface of sea and, hence, from the rig or platform servicing them.
Examples of known tree and wellhead assemblies are disclosed in US 2005/0121198, U.S. Pat. Nos. 6,581,691, 6,547,008 and 5,544,707.
US 2005/0121198 discloses a subsea production system having a Christmas tree. The tree comprises a vertical bore with a hub at its upper end. Production and annulus valves are connected to the tree by means of separate valve blocks.
U.S. Pat. No. 6,581,691 discloses a landing adapter for soft landing a tubing hanger in the bore of a production tree or wellhead housing. Again, U.S. Pat. No. 6,581,691 discloses a wellhead tree having an integral connection hub. A plurality of valves are provided, some of which are internal to the tree and others of which are external and connected to the tree by way of one or more valve blocks.
A wellhead tree with a vertical bore and integral connector hub is disclosed in U.S. Pat. No. 6,547,008. The tree is provided with production and annulus valves that are external to the tree and bolted to the outer wall of the tree. A similar arrangement is shown in U.S. Pat. No. 5,544,707.
Further, EP 0 719 905 and EP 0 989 283 both describe a particularly advantageous horizontal tree and wellhead assembly. The wellhead described comprises a wellhead housing, to which a spool tree is connected by means of a conventional connector assembly. The spool tree is of a generally vertical arrangement, having a central longitudinal bore therethrough of relatively large diameter. A production port extends laterally from the central bore through the wall of the spool tree. A production valve block, typically including a production wing valve, is bolted to the side of the spool tree body so as to communicate with the production port. The upper end of the spool tree body terminates in a vertical intervention hub, to which may be connected a BOP using a conventional drilling connector. A set of annulus valves are also connected to the outside of the spool tree body, so as to communicate and align with a series of annulus ports extending through the spool tree body and communicating with the central bore of the tree.
More recent developments to the concept of the horizontal tree disclosed in EP 0 719 and EP 0 989 283 are described in U.S. Pat. No. 6,050,339. The horizontal tree disclosed in U.S. Pat. No. 6,050,339 comprises a vertically oriented longitudinal central bore. The tree is connected at its lower end to a wellhead housing. The upper end of the tree body is formed as a connector or hub, for connection to a BOP and a drilling riser or the like. A production port extends laterally through the body of the tree from the central bore. A production valve is incorporated into the tree body for controlling the flow of production fluids from the well. Similarly, annulus ports extend through the tree body to communicate between the central bore and the exterior. Valves incorporated into the tree body control the flow of fluid through the annulus ports.
U.S. Pat. No. 6,470,968 discloses a subsea tree and tubing hanger system. The subsea tree is of the horizontal arrangement, having a vertically oriented central bore, from which extends a lateral production port through the body of the tree to a production valve bolted onto the exterior of the tree body.
More recently, US 2004/0112604 discloses a horizontal spool tree with improved porting. Again, the tree comprises a vertically oriented central bore. The tree body is connected at its lower end to a wellhead housing, while its upper end forms a connector or hub for connection to a riser, BOP stack or the like. A lateral production port extends from the central bore through the tree body. A production flow valve is included in the tree body to control the flow of production fluids. However, a second production valve for controlling the production fluid flow is housed in a block bolted to the exterior of the tree body. Similarly, annulus ports are provided through the tree body, with an integral flow valve. However, a second annulus flow valve is located in a block bolted to the exterior of the tree body.
Since their inception in the early 1990's, a significant number of horizontal trees have been built and installed, in particular in subsea wellhead installations, as a result of the advantages they offer over the conventional vertical tree design. However, a number of problems remain with horizontal trees.
As noted above, horizontal trees are formed with a central, vertically arranged tree body, through which extends the central bore. A horizontal production port and various annulus ports are provided in the tree body. The lower end of the tree body may be formed as an integral connector for securing to a wellhead housing. The upper end of the tree body is invariably formed as a connector or vertical intervention hub, to allow connection to a BOP stack, riser or the like. In view of the wide range of rig and platform assemblies used to service subsea wellheads, the tree body needs to be formed with an upper connector or hub of considerable length, in order to accommodate the variety of connection designs that may be employed. As a result, the body of a horizontal tree is a massive component, the size of which limits the ability to machine and form other features or components. Accordingly, the need to provide the necessary production, annulus and cross-over valves as bolt-on components arises. The massive size of the tree body also limits the extent to which other features may be incorporated into the tree body, such as certain flowloops or cross-over lines.
The inability to include the aforementioned features into the horizontal tree body may detract from the integrity of the tree. This is particularly the case if the production flow path of the known horizontal tree designs is considered. As noted above, it has been the case that the valves necessary to control the production fluid flow cannot necessarily be accommodated within the tree body. Thus, production flow valves have been incorporated using bolt-on valve assemblies. This results in a potential fluid leak path at every joint of the bolt-on assemblies. The potential for a fluid leak is particularly high at the very high fluid pressures encountered in the production flow path, in which fluid is flowing at full well pressure before reaching the production choke. Indeed, an analysis of the known horizontal tree designs shows that the highest integrity of the horizontal tree body extends along the central longitudinal (vertical) bore and not along the production flowpath. While this is of advantage during downhole operations where entry to the well is required through the central bore, such operations occupy only a very minor portion of the working life of the wellhead assembly, with the majority of the time being spent with the well in production mode. Accordingly, the majority of the operations conducted through a wellhead assembly with a horizontal tree will involve using a fluid flowpath through the assembly that comprises one or more potential fluid leak paths.
There is a need for a design of wellhead assembly and horizontal tree that incorporates an improved integrity in the fluid flowpaths through the tree.